IMHO it is extremely harsh how the self-signed certs are treated by
OpenLDAP. In the majority of cases this is forcing people (after many
hours of struggling) to use "TLS_REQCERT never" or similar settings,
which ends up being a lot more insecure than it would be to accept a
known self-signed cert... Not to mention that the syncrepl suboption
"tls_reqcert=never" is apparently ignored so practically I've found
that syncrepl is currently inoperable with any form of encryption.
Is there anybody who could tell me what this is good for?

Interestingly, plenty of people have gotten this to work. First, you need
to know how to create self-signed certs using a CA. Of course, that's
really off-topic for the OpenLDAP list, even though it has been discussed
many times. But until you know how to get that working, you won't be able
to get the syncrepl client to work, either.